1. Field of the Invention
This invention relates to a method for manufacturing a fluorescent substance containing nitrogen, to a fluorescent substance containing nitrogen, and to a light-emitting device employing the fluorescent substance containing nitrogen.
2. Description of the Related Art
Nowadays, there is an increasing trend to employ a white light-emitting diode (white LED) in various fields. The white LED emits white light in such a manner, for example, that several kinds of light to be emitted from several kinds of fluorescent substances each employed for shifting the wavelength of part of light emitted from an LED chip are mixed with the light that has been emitted from an LED chip without the wavelength thereof being shifted at all so as to obtain white light. In the conventional white LED however, since it is difficult to obtain the light of long wavelength side in the visible radiation region, the light emitted from the conventional white LED becomes somewhat yellowish white. Because of this, this white light is insufficient in color tone if it is to be employed as an illumination for display application or for medical application, so that there is a strong demand for an LED which is capable of emitting a somewhat reddish white light.
As for the fluorescent substance which is capable of emitting a reddish light when an ultraviolet or blue light-emitting diode is employed as a light source, there has been proposed a nitride fluorescent substance having a composition represented by MXSiYNZ:Eu (wherein M is at least one selected from the group consisting of Ca, Sr, Ba and Zn; Z=(⅔)X+( 4/3)Y; preferably X=2 and Y=5 or X=1 and Y=7). Since this fluorescent substance is capable of absorbing light having a short wavelength ranging from 300 to 550 nm and capable of emitting a light having a long wavelength ranging from 550 to 750 nm, it is possible, through the employment of this fluorescent substance, to emit lights ranging from yellow to red from the light of ultraviolet region while absorbing visible lights of indigo, blue and bluish green.
The aforementioned nitride fluorescent substance can be manufactured in such a manner that a base material of fluorescent substance, and a refined metal or a nitride thereof acting as an activator are mixed together and then subjected to sintering in an ammonia atmosphere using a boron nitride crucible at a temperature ranging from 1200 to 1600° C. In this case, since an apparatus for disposing ammonia employed as an atmospheric gas is required to be installed and moreover, since the process is complicated and a manufacturing apparatus of large scale is required to be employed, the cost for manufacturing the nitride fluorescent substance would become high. Furthermore, if an LED which is capable of emitting white light is to be manufactured by using this nitride fluorescent substance, another fluorescent substance which is capable of emitting yellow light or blue light is required to be employed in combination with this nitride fluorescent substance.
As described above, the conventional nitride fluorescent substance is accompanied with problems that it is difficult to manufacture a fluorescent substance exhibiting useful light-emitting properties and that it is difficult to determine a suitable mixing ratio for the adjustment of white-emitting light.